Aluminum alloy



Patented Mar. 3%, 1937 UM! ALILGY Walter Bonsack, Cleveland Heights, and .lohn G.

G. Frost, Cleveland, Ohio, assignors to The National smelting Company, Cleveland, Uliio, a

corporation of @hio No Drawing. Application August 17, 1936, Serial No. 96,519

'7 Claims.

This invention relates to alloys, and more particularly aluminum base alloys having a low thermal expansion and high thermal conductivity.

This application is a continuation in part of our 5 application Serial No. 43,635 filed October 4, 1935. In the making of cast articles, such as heat transfer elements, ylinder heads and similar articles utilized in internal combustion engines and motor constructions, it is desirable to make 0 such articles from aluminum and alloys thereof because of their low specific gravity.

.While aluminum and aluminum alloys in general have a high thermal expansion, some aluminum alloys have been developed with a considerably lower thermal expansion, making them more desirable for use in the manufacture of castings for motor parts.

It has been found, however, that, in addition a to having a low thermal expansion, aluminum alloys for use in making motor part castings should also have a high thermal conductivity, as well as good mechanical and physical properties, such as ductility, machinability and suitable strength at low temperature or at elevated temperatures occurring in internal combustion engines.

It is, therefore, an object of this invention to provide a light weight aluminum base alloy having a comparatively low thermal expansion and a comparatively high thermal conductivity, and also having suitable mechanical and physical properties for the casting of motor parts and other similar articles requiring similar properties.

Another object of this invention is to provide such an aluminum base alloy which is readily machinable, which may be readily cast in the usual type of molds, and which has a fine, homogeneous grain structure, and which may be subjected to elevated temperatures without causing loss of strength.

While certain aluminum-silicon alloys are known to have a relatively low thermal expansion and relatively high thermal conductivity as compared to other aluminum alloys, it is necessary with these aluminum-silicon alloys to have the other desirable properties of ductility, machinability and good bearing qualities to make them more suitable for the production of bearings, heat transfer elements for use at low temperatures such as parts for refrigerating apparatus and for use at high temperatures such as cylinder heads and other internal combustion engine parts. It is an object of this invention to provide such an aluminum-silicon alloy with such desirable mechanical properties and still maintain the relatively low thermal expansion and relatively high thermal conductivity of these alloys.

It has been discovered that if tin is added in proper proportions to aluminum base alloys containing silicon it will increase the thermal conductivity without materially aifecting the thermal expansion of the alloy. Tin also improves the machinability of the alloy, and provides better bearing qualities in castings made therefrom.

Castings, such as cylinder heads, heat transfer parts, or motor parts, which are subjected to low or elevated temperatures may, therefore, be produced from an alloy of aluminum, silicon, tin

and zinc and have excellent properties for the manufacture of articles of this nature.

In preparing our improved aluminum base alloy, silicon is used as the predominating ingredient and may be present in amounts ranging from 7% to 15%. The higher percentages of silicon are more effective in reducing the thermal expansion of the alloy. It is desirable, however, to avoid the use of too large a proportion of silicon, since it has a tendency to segregate out in large crystals, unless the casting is chilled very rapidly. It is, therefore; preferable to use silicon in an amount from about 8% to 13%r Tin is an important addition to the aluminum silicon alloy in order to provide better thermal conductivity and bearing qualities, as well as to improve its machinability. In order to obtain these properties, tin may be present in the amount of .3% to 3%, and preferably in an amount from .5% to 1.5% or 2%. Tin, if present in too large amount, has a tendency to segregate and it is not desirable to have more than about 3% tin in the alloy.

When present in small amounts up to approximately 3%, zinc has the effect of improving the mechanical properties, such as increasing the tensile strength and hardness of the metal and improving machi ability. When added in amounts such as 2 o to 3% it tends to reduce the thermal conductivity of the alloy and to increase the thermal expansion. If relatively high thermal conductivity and relatively low thermal expansion are desired, it is preferable to have zinc present in amounts less than 2%, such as 5% to 1.5%, and when present in these amounts the thermal conductivity and thermal expansion of the alloy are not materially afiected.

Copper may be present in the alloy in small amounts, for in amounts such as .6% or less it does not seem to materially affect the thermal expansion of the alloy, and it is, therefore, not necessary, in making the alloy, to exclude base metals containing small amounts of copper. If copper is present in the alloy in the amount of approximately 1% or more, it has the effect of substantially reducing the thermal conductivity of the alloy.

If relatively high thermal conductivity of the alloy is desired, copper should not be present in an amount more than approximately .2%, but when such a high degree of thermal conductivity is not necessary, the copper may be present in amounts up to about .6% or more.

Chromium, manganese, nickel, cobalt and titanium tend to reduce the thermal conductivity of aluminum-silicon alloys. These metals also have a tendency to form compounds with aluminum at a high melting point, and such compounds tend to segregate from the molten metal before solidification occurs. When a relatively high thermal conductivity of the alloy is desired, the above metals should not be present in an amount sufficient to materially reduce the thermal conductivity, and. if present at all, should only be present in such minimum amounts as are practicable in the commercial manufacture of the alloy, or as will not reduce the thermal conductivity beyond the limit desired for the alloy. Iron also has the tendency to reduce thermal conductivity, but iron, as is well known, is usually present'as an impurity in aluminum or aluminum alloys. However, it should not be present in an excessive amount, such as 1% or more.

Many types of castings where rapid heat transfer is desired, such as for compressors, bearings, motor parts and other castings which are subjected to elevated temperatures, or even for refrigerator parts and other uses where the castings are used at low temperatures, can be advantageously made of an alloy of aluminum, silicon, tin and zinc.

As illustrations of our approved alloys, the following specific'examples are given.

An alloy containing 12 to 13% silicon, 1%

tin, 1% zinc, and the balance aluminum and minor impurities was chill cast and aged for eight hours at 175 C. Upon being tested it was found to have a thermal expansion of 20x10- inch per inch per degree C. between a temperature range of 20 and 100 C. It also had a thermal conductivity of .35 calorie per square centimeter per second at 30 C. The tensile strength is approximately 21,000 pounds per square inch, the elongation 4.4% in 2",.and it had a hardness, Rockwell E, of 44.

The same alloy similarly aged and with approximately-2% tin instead of 1% was found to have substantially the same expansion and thermal conductivity. A tensile strength of about 20,500 pounds persquare inch, an elongation of 3.3% in 2" and a hardness, Rockwell E of about 45. It may be noted that the higher percentages of tin considerably improve the machinability of these alloys.

An alloy containing approximately 7.5% silicon, .5% tin and 1% zinc and the balance aluminum and minor impurities was chill cast and aged for eight hours at 175 C. Upon being test?" ed it was foundto have a thermalexpansion of 21.5 10 inch per inch per degree C. between a temperature. range of 20 and 100 C. and a thermal conductivity of .38 calorie per square centimeter per second at 30 C. The tensile' strength was approximately 20,400 pounds per square inch, the elongation 5.8% in 2" and hardness, Rockwell E, 38.

The same alloy similarly aged and with 2% zinc instead of 1% had a thermal expansion of 21.8 10- inch per inch per degree C. between a temperature range of 20 and 100 C., and a thermal conductivity of .37 calorie per square centimeter per second at 30 C. It had a tensile strength of 20,600 pounds per square inch, an

elongation of 4.7% in 2", and a hardness of, Rockwell E, 39.

An increase in the percentage of zinc in the composition has the effect of improving the machinability of the alloy. The improvement in machinability is not quite as great. as that attained by increasing the percentage of tin. The desired machinability can often be obtained, however, at less expense by increasing the percentage of zinc rather than by increasing the percentage of tin.

The properties of the alloy particularly with respect to thermal conductivity and ductility, may be varied by suitable heat treatment. For example, an alloy containing approximately 7.5% silicon, 2 tin and 1% zinc and the remainder alluminum with minor impurities when aged for 8 hours at 175 C. had a thermal expansion at 21.8X10- inch per inch per degree 0. between a temperature range of 20 and 100 C., and a thermal conductivity of .38 callorie per square centimeter per second at 30 C. It had a tensile strength of 19,800 pounds per square inch, and an elongation of 3.5% in2", and a hardness of, Rockwell E", 41.

The same alloy annealed for 2 hours at 450 C. and cooled had a thermal conductivity of .41 calorie per square centimeter per second at 30 C., a tensile strength of 16,800 pounds per square inch, an elongation of 8.3% in 2", and a hardness of, Rockwell E, 12, with substantially the same thermal expansion.

As previously stated, our invention contemplates that copper may be present in small amounts in the alloy and it is also to be understood that minor impurities may be present without departing from the scope of the invention set forth in the claims.

Aluminum base alloys having low thermal expansion and high thermal conductivity, and containing aluminum, silicon and tin, are described and claimed in our copending application, Serial No. 96,517, filed August 1'7, 1936.

Furthermore, it will be understoodthat the present invention is not limited to the specific details set forth in the foregoing examples, which should -be construed as illustrative, and not by way of limitation, and in view of the numerous modifications which may be effected therein without departing from the spirit and scope of this invention, it is desired that only such limitations be imposed as are indicated in the appended claims.

What we claim is:

1'. An aluminum base alloy having a low coefficient of expansion and high thermal conductivity, comprising 7% to 15% silicon, .3% to 3% tin, .5% to 3% zinc, and the balance substantially all aluminum.

2. An aluminum base alloy having low "*-2%tin, .5% to 2% zinc, and the balance substantially all aluminum.

3. An aluminum base alloy having a low co-- efliclent of expansion and high thermal conductivity, comprising about 8%v to 13% silicon, .3% to 3% tin, .5% to 3% zinc, and the balance substan- 6. An aluminum base alloy having a low cotially all aluminum. efilcient of expansion and high thermal conduc 4. An aluminum base alloy having a low 00- tivity, comprising 7% to 15% silicon, about .5% eflicient of expansion and high thermal conducto 2% tin, .5% to 33% Zinc, and the balance sub- 5 tivity, comprising about 8% to 13% silicon, about stantially all aluminum. I

.5% to 2% tin, .5% to 2% zinc, and the balance 7.Achill casting formed from an aluminum base substantially all aluminum. alloy having a low coefllcient of thermal expan- 5. An alumimun base alloy having a low' cosion and high thermal conductivity, comprising eflicient of expansion and high thermal conducabout 8% to 13% silicon, .5% to 3% tin, .5% to 3% 10 tivity, comprising about 8% to 13% silicon, about zinc, and the balance substantially all aluminum. l0

1% tin, about 1% zinc, and the balance substan- WALTER BONSACK. tially all aluminum. JOHN G. G. FROST. 

